1. Introduction
Hybrid circuit boards, including those with three-dimensional features, may be coated with photoresists during various stages of manufacture. The present invention discloses a means of applying an electrodepositable resist material of uniform thickness which gives a pinhole free coating that conforms to three-dimensional surfaces. Another advantage of this method is the elimination of hazardous solvent developers.
2. Discussion of the prior art
Hybrid circuits are circuits in which chip devices of various functions are electrically interconnected on an insulating substrate on which conductors or combinations of conductors and resistors have previously been deposited. Hybrid microcircuits are used in applications where high density, low volume, and low weight are required for an electronic system. Without these constraints, printed wiring boards with solder-attached, pre-packaged components are more economical. Because of the advantages of weight and volume which they have over printed wiring assemblies, hybrids have found extensive use in airborne and space applications. Other applications include communication systems, high-speed computers, guidance and control systems, radar systems, heart pacemakers, other medical electronic devices, and automotive electronics.
The resistors used in hybrid manufacture are batch processed therefore fewer interconnections need to be made. The reliability of hybrids over printed circuits may also be increased by various methods. Resistors can be trimmed statically or dynamically to precise values with close tracking; hence, precision circuits can be fabricated that are not possible with printed circuits. From a thermal management standpoint, hybrid circuits have the advantage of direct mounting of high-power devices on a thermally conductive ceramic which is greatly superior to mounting pre-packaged components onto a thermally-insulative epoxy or polyimide circuit board. To remove heat from a plastic printed circuit board, heavy metal heat rails must be attached with adhesive or metal-core boards must be used as substrates.
A detailed description of the processes for manufacturing hybrid circuit boards may be found in "Hybrid Microcircuit Technology Handbook--Materials, Processes, Design, Testing and Production", James J. Licari and Leonard R. Enlow, Chapters 2-7, pages 25-246, Noyes Publications (1988).
Substrates for hybrid board manufacture are usually ceramics made from alumina. The first step in the manufacturing process is the deposition of metals or metal compounds onto the substrate. These metals ultimately provide the conductor and resistor patterns and functions. Typically a substrate is coated sequentially with a layer of resistive material, a barrier metal, and a top layer of conductive metal all applied by vacuum evaporation, sputtering or some other method capable of depositing a thin coating on the order of 200 Angstroms to 20,000 Angstroms.
Use of photoresists in the manufacture of hybrid boards allows selected areas of a surface to be removed at various processing steps leaving other areas (protected by the photoresist) as defined patterns of metal conductors, resistors, dielectrics, or inorganic passivation layers. Photoresist materials used in fabrication of hybrid boards may be of two types: those that on exposure to light are cross-linked forming a hardened coating which is resistant to etchants and other solutions (negative) and those that on exposure to light are decomposed and can be dissolved selectively (positive). The photoresist coating is applied over the entire surface of the substrate, soft-baked to remove solvent, then exposed to ultraviolet light through a mask.
Liquid photoresists are generally applied to substrates as a single unitary coating by spraying, dipping, spin coating or roller coating. Resultant coatings may show variations in thickness due to build-up at the outer edges. Any blemishes, pinholes or other imperfections in the hardened resist become sites for the penetration of etchants or plating solutions, resulting in opens or shorts in the circuitry.
For three-dimensional boards, there is the added problem of obtaining a uniform coating particularly at holes and corners, which leads to different exposure and developing times and often results in defective products. Another type of photoresist sometime used is referred to as dry film. While dry film resists have been used in the preparation of two-dimensional boards, they do not lend themselves to application in three-dimensional systems because dry film is generally applied using heat and pressure with pinch rollers that do not readily conform to three-dimensional features.
The photoresist that remains on the substrate must also provide good adhesion qualities and be able to withstand subsequent fabrication steps such as electrolytic plating with metals such as copper, nickel or gold.
U.S. Pat. No. 4,167,413 discloses a method of making hybrid integrated circuits with photoresist laminate. The process involves use of a preformed piece of dry film photoresist which is laminated to the substrate by simultaneously applying pressure and heat. This method provides no protection for the edges and requires an additional step of painting a layer of liquid resist around the edges where the coating need not be uniform. Such a process would not be suitable for use with three-dimensional substrates.
U.S. Pat. No. 4,412,377 discloses a method of making hybrid integrated circuits with photoresists using a typical printing process. The resist is printed on the ceramic substrate in the desired pattern and dried at about 125.degree. C. The resist layer is subsequently removed using trichloroethylene solvent. This solvent must be used with adequate ventilation and may have health and environmental effects. Again, such a process is not applicable to three-dimensional substrates.
U.S. Pat. No. 4,554,229 discloses a method of making multilayer hybrid integrated circuits using a dry film photoresist that is removed using methylene chloride.
U.S. Pat. No. 4,756,756 describes a method of forming thick-layer, hybrid electronic printed circuits in which a negative photoresist is deposited uniformly over the surface by spraying. The image-wise exposed resist is subsequently removed using pure xylene.
It is an object of the present invention to provide a means of manufacturing an improved hybrid integrated circuit using photoresist materials which may be applied by a method applicable to all geometries of substrates. The applied resist is preferably aqueous developable and strippable providing for improved environmental handling.